As the title says it all, what do you pro's suggest me to do? I'm currently shooting the 100-400 mm IS on 50D, and am more or less satisfied about the results (aviation spotting), but as it has had its best time I'm looking for an alternative.

Both the 300 mm and 400 mm primes crossed my mind, but I miss -or mm for the 300 mm or IS for the 400 mm- .So, would it be wise to buy a 400 mm prime f5.6 non-IS, or 300 mm f4 prime IS with extender now.. or just keep the 100-400 mm IS a bit longer until rumors become reality at an affordable price tag (max would be $3000)?

Note: I love playing with the photoshop elements 'cropping tool', so which one gives the best results for some heavy cropping?

We had the 400/5.6 and the 100-400/5.6 IS in here to evaluate. The image quality of the 400 was better, but I was outvoted and we got the zoom lens instead. The lure of zoom and IS was deemed more important than the better image quality.... though to be fair, it was a noticeable difference when you pixel peeped, but it was not what I would call a significant difference.

No experience with the 300f4 and teleconverter...

My recomendation would be to find a camera shop with your three choices and go try them out and do some comparison shots.... bring a tripod for comparing image quality and try hand-held shots to see what effect IS (or lack of) has on you. Some people have steadier hands than others so IS is less of an issue for them, but in the end it is a personal choice based on your needs...

I'll be waiting for reviews of the new Sigma 120-300/2.8 + tc. Might be a tad higher than your price point but that seems like the only thing filling in the gap between < $2k "telephoto" and >$4k "!!TELEPHOTO!!"

I've had the 300mm f/4, the 100-400mmL and the 400 f/5.6 L. All are fine lenses, but I kept the 100-400mmL. Sharpness at 400mm is pretty much identical between the 400mm and the zoom. The 300 plus TC was not up to matching the native lenses. This has been confirmed over and over, with the occasional user seeing it different, likely due to a bad sample.The big advantage of the 100-400L and the 300mmL is their much greater magnification. If you are photographing small birds like humming birds, you need a close focusing lens. If you are photographing birds in flight at a long distance, thats where the 400mm f/5.6 shines.

Hi Tron, do you mean the 100-400 gives the same results as the 300 with extender III ?

And a question that still plays : I use a 50D, that auto-adds a crop of 1.6.. with a 1.4 III extender extra, do I loose more than 1 stop? (300mm F4 + extender III + 50D = f5.6 or .. higher? AND does it still has AF?)

Hi Tron, do you mean the 100-400 gives the same results as the 300 with extender III ?

And a question that still plays : I use a 50D, that auto-adds a crop of 1.6.. with a 1.4 III extender extra, do I loose more than 1 stop? (300mm F4 + extender III + 50D = f5.6 or .. higher? AND does it still has AF?)

Gert.

No, he means the 100-400 is better at one thing (>), but the 300+1.4xIII is better at another (<). Read the "Greater Than" (US English...) or "Less than" signs (> & <) as "Better Than" and "Lesser Than".

As for the second question, no, the crop has no effect on the f/stop of the lens. 5.6 is still 5.6...

Regarding the question of which to get, that depends on if you want the versatility of the 100-400, over just 300 & 420mm, or if you really want the better image of the 300 straight up. Unfortunately, about all that waiting for a revamped lens will get you is a much lighter wallet, if Canon's recent history is any indicator! Oh, and prbably a better image, but...

I recently tested the 400L and 100-400 and I found that for shots faster than 1/500 sec, the 400L gave me sharper images so I bought the 400L... I agree with Mt Spokane that this lens shines for birds in flight but far I have used it successfully for still images of flowers, celestial bodies and of course "still" wildlife.... of course wildlife never sits still....

Correct me when wrong, but that review seems from 2009, and probably tells the story about the extender I or II version.. the one I want is the III version and should not eat away quality.. or am I wrong?

The reviews of the II and III TCs show very little improvement in resolution - look at The-digital-picture.com iso tests, where you can see the effects of both TCs on the 300 f/4. The Series III were introduced to pair with the Series II supertelephotos where they talk better to the lens and the camera for better autofocus.

My copy of the 200-400mm is as sharp at the centre, but not the edges, as the 400m L, which I sold.

+1 I have the 300 f/4 IS and as a stand alone it is great. But it's IQ is killed with the 1.4X. Frankly, none of my lenses are benefited by the extender and I am considering selling it. I think it is only good for 2.8 teles or f/4 superteles. As I have neither, it is of little use to me presently...